The present invention relates to a process for blending two or more petroleum oils to mitigate fouling of refinery equipment, particularly of heat exchangers, by avoiding nearly incompatible blends.
It is well known that petroleum crude oils and asphaltene containing oils derived from petroleum crude oils have the tendency to deposit organic solids, called foulant and coke, on refinery process equipment that contact the oil. Such process equipment include, but are not limited to, pipes, tanks, heat exchangers, furnace tubes, fractionators, and reactors. Even small amounts of foulant or coke results in large energy loss because of much poorer heat transfer through foulant and coke as opposed to metal walls alone. Moderate amounts of foulant and coke cause high pressure drops and interfere with and make process equipment operate inefficiently. Finally, large amounts of foulant or coke plug up process equipment to prevent flow or otherwise making operation intolerable, requiring the equipment to be shut down and cleaned of foulant and coke.
It is also well known that petroleum derived, asphaltene containing oils that have undergone reaction at high temperatures, above 350.degree. C., have a tendency for rapidly fouling process equipment, either on cooling or by blending with a more paraffinic oil. Such processed oils include, but are not limited by, the highest boiling distillation fraction after thermally or catalytically hydrothermally converting atmospheric or vacuum resid of petroleum crude and the highest boiling fraction of the liquid product of fluid catalytic cracking, called cat cracker bottoms or cat slurry oil. This rapid fouling is caused by asphaltenes that become insoluble on cooling or on blending with a more paraffinic oil. Here asphaltenes are defined as the fraction of the oil that is soluble when the oil is blended with 40 volumes of toluene but insoluble when the oil is blended with 40 volumes of n-heptane. If the asphaltenes become insoluble at high temperatures, above 350.degree. C., they rapidly form toluene insoluble coke (see I. A. Wiehe, Industrial & Engineering Chemistry Research, Vol. 32, 2447-2454.). The previous patent application of Wiehe and Kennedy disclosed that the mere blending of two or more unprocessed petroleum crude oils can cause the precipitation of insoluble asphaltenes that can rapidly foul process equipment or when such crude oil blends are rapidly heated above 350.degree. C., the insoluble asphaltenes can coke pipestill furnace tubes. If the blending of oils causes the precipitation of asphaltenes, the oils are said to be incompatible as opposed to compatible oils that do not precipitate asphaltenes on blending. Thus, incompatible blends of oils have a much greater tendency for fouling and coking than compatible oils. Once an incompatible blend of oils is obtained the rapid fouling and coking that results usually requires shutting down the refinery process in a short time. This results in a large economic debit because while the process equipment is cleaned, large volumes of oil cannot be processed. In the past most refineries have learned by trial and error to avoid certain crude oils or not to blend certain processed oils or to reduce the severity of the process in order to make more blendable process oils.
The blending of oils in a refinery is so common, especially for crude oils, that few, if any, refineries can be economically viable without blending oils. This is both done to be able to produce the most economical range of products and to handle the multiple feedstocks at a refinery that arrive at similar times with limited number of storage tanks.
Now, it has been discovered that compatible, but nearly incompatible, blends of oils can foul metal surfaces, particularly heated metal surfaces, at a higher rate than the individual oils in the blend. While the rate of fouling is the highest for incompatible blends, this rate is high for nearly incompatible blends but it decreases as proportions of the blend are selected to be farther and farther from those proportions that result in incompatibility. It also has been discovered that the same oil compatibility model that enabled predicting the proportions of oils in a blend that avoid incompatibility, as taught in the previous application, also enable predicting the proportions of oils in a blend that avoid near incompatibility, albeit using a different criterion.